This invention relates to alignment of a radiation receptor element, such as a CCD photodetector array, behind a lens as in a sensor of an imaging system, wherein the lens directs rays of radiation to the radiation receptor element and, more particularly, to the alignment of an array of photodetectors by use of a sine wave grating located adjacent the lens to produce a multiple-peak diffraction pattern in a back focal plane of the lens, wherein the peaks serve as benchmarks for locating the photodetectors. Electronic imaging systems are frequently constructed of an array of radiation-sensitive elements, such as photodetectors, located behind a lens, at the back focal plane of the lens. Rays of radiation from a distant subject propagate towards the lens, and are focused by the lens upon the array of photodetectors. The photodetectors output electrical signals in response to the incident radiation. The electrical signals are processed, typically, by use of multiplexing and scanning circuitry which collects and samples the signals for further image processing, which may include filtering and storage for later use, such as for displaying an image of the subject. In order to display a sharp image of the subject, it is important that the photodetectors be positioned accurately in the focal plane of the lens.
A problem arises in that the need for accurate positioning of the photodetectors produces a burden in a process of manufacturing an imaging system. By way of example of such manufacture, the array of photodetectors may be fabricated as a CCD (charge coupled device) array configured as a CCD module. For a large array, a plurality of the CCD modules can be located contiguous each other. Alignment of a CCD module in respect to the lens has been accomplished by use of a microscope and a reticle to align the CCD module to reference pins. The CCD module is then coupled to the lens via the reference pins. The alignment has been accomplished about two perpendicular coordinate axes, X and Y, in the focal plane, and has included rotation about a Z axis perpendicular to the focal plane. Preferably, the alignment should make provision for translation about three perpendicular coordinate axes and rotation, or tilting, about the three perpendicular coordinate axes. A supporting structure of the imaging system supports both the CCD module and the lens in their respective positions. The foregoing alignment process has required removal of the lens from the supporting structure. The alignment also provides for adjustment of the CCD module in the Z direction so that, upon inserting the lens in its place in the supporting structure, the CCD module is located properly in the back focal plane.
In the foregoing example of an alignment process, a flange of the lens has been used as a reference plane for alignment in the Z direction. However, in the event that there are variations of focal length between lenses wherein the variation is bigger than a depth of focus of each lens, then the position of the CCD module may not be optimum upon replacing one lens with another lens. Furthermore, since the foregoing alignment process has been accomplished with the lens removed from the supporting structure, there is a possibility that an optical axis of the lens may not be truly perpendicular to the front face of the CCD module due to lens aberrations or machining tolerances in the flange of the lens.